This invention relates to an image memory control device for a video signal special reproduction system and, more particularly, to an image memory control device which is suitable to be adapted for a video tape recorder (to be referred to as a VTR hereinafter) for performing noiseless special reproduction.
As is well known, in a helical scan type VTR, helical video tracks are formed on a magnetic tape to be inclined with respect to the travel direction. In the VTR of this type, special reproduction is performed by driving the tape at a speed different to that in the recording mode, thereby allowing slow-motion reproduction or high-speed reproduction. Alternatively, the tape is stopped and a still image is reproduced. In such special reproduction, the video head of the VTR may sometimes not trace a video track of the tape correctly, but helically trace over a plurality of video tracks simultaneously, thus leading to noise in the image.
Various countermeasures are conventionally proposed to prevent noise. One of the countermeasures uses a field memory.
For example, when slow-motion reproduction is performed by continuously running the tape at 1/4 speed of that in the recording mode, every 8th field can provide an output which is substantially completely free from noise. The reproduced output of the field free from noise is stored in a field memory. Thereafter, when a field subject to noise or degraded image quality is to be reproduced, the signals stored in the memory are read out and are output instead of the reproduced output, so that a special reproduction image without noise can be obtained.
When a special reproduction image is obtained using a field memory of this type, the continuity of the signals on the screen must be maintained.
In an NTSC television signal, one field consists of 262.5 horizontal scan lines. Therefore, in a still image reproduction, when the memory signals are read out in units of fields, a skew of 1/2 H (H: horizontal scan line) appears between two adjacent fields on the screen.
In a home VTR, two video heads having different azimuth angles are mounted on a rotational disk at an angle of 180.degree. from each other. 1-track 1-field video signals are recorded on a magnetic tape travelling at a predetermined speed such that video tracks of every other field are formed helically on the magnetic tape.
In the home VTR of this type, video signals are recorded on the magnetic tape such that a positional shift of horizontal sync signals, called H, is formed between adjacent video tracks. When the video heads helically trace the video tracks in the manner as described above, the number of horizontal scan lines in the signals reproduced during a half rotation period of the disk, i.e., the signals reproduced by one video head, deviates from the correct value. For example, the number of horizontal scan lines of one field of NTSC television signals deviates from 262.5. This number varies according to the travel speed of the magnetic tape (FIG. 3).
Therefore, even during special reproduction other than the still image reproduction described above, such as slow-motion reproduction or high-speed reproduction, if only a field memory is used, a skew in accordance with the tape travel speed appears on the screen. In this case, a skew of 1/2H or more poses a practical problem.
In this manner, since the number of horizontal scan lines changes in accordance with the tape travel speed, a large skew (1/2H in the worst case) sometimes appears on the screen. In other words, some values of a tape travel speed cannot be practically used due to undesirable skew.
In a special reproduction system using a field memory, optimum skew correction must be performed in accordancewwith the tape travel speed.